A Novel User-Controlled Assisted Standing Control System for a Hybrid Neuroprosthesis

Abstract

Spinal cord injury (SCI) is a serious condition with 17,000 new cases each year and an estimated total of 282,000 people in the United States who have SCI. Some people with SCI who have paraplegia suffer from paralysis, muscle spasticity, bone changes, chronic pain and other problems. Active orthoses such as the ReWalk, EXPOS, and Ekso have improved the quality of life of people with SCI. The hybrid neuroprosthesis is an active orthosis that uses functional electrical stimulation (FES) at the quadriceps and has two main purposes: restoring mobility in people with SCI and providing physical therapy for the user outside of a clinical setting. To mobilize people with SCI, the neuroprosthesis must provide assisted movement for a sitting to standing motion. A standing control system developed by the Pitt Neuromuscular Control and Robotics Laboratory (NCRL) before this proposed system did not give enough control of the movement to the user and FES alone did not provide enough torque at the knees for standing. The NCRL neuroprosthesis was modified to include a harmonic gearmotor at the knees, a thumb joystick for user control, and a force sensing walker. A control system using a finite state machine (FSM) was designed to perform hybrid standing in the neuroprosthesis. The FSM is divided into 3 states and uses 5 separate controllers: a tracking controller for forward leaning during sitting, a tracking controller to synchronize the knees, a tracking controller to lock the knees during standing, a hip tracking controller, and openloop FES. Four experiments were performed on subjects to analyze control performance, power usage, and energy consumption during motors only and hybrid standing. A subject with SCI successfully performed several trials of hybrid standing. The controllers performed sufficiently accurately, and several minor control problems were fixed. The highest average energy consumption at the knee motors was 88.4 joules during experiment 1. The hybrid standing experiment demonstrated a modest energy reduction of 15% in a subject with SCI. The hybrid standing demonstrated a high energy reduction of 74% in the right knee in experiment 2, through hybrid actuation and a slower standing speed.